Field of the Invention
The present invention relates to a switched-mode power supply having a switch for the pulsed application of a variable supply voltage to a primary coil of a transformer and having a first current measuring configuration for providing a current measurement signal dependent on a current through the primary coil.
Switched-mode power supplies of this type generate from the supply voltage one or more stabilized DC voltages for supplying loads connected to a secondary side of the transformer. The supply voltage is generated from an AC mains voltage by rectification and smoothing, the switched-mode power supplies being configured to function over a wide mains voltage range, e.g. 90V . . . 270V. In the event of a supply voltage produced from a mains voltage which is below the configuration range, the regulating behavior is impaired and there is the risk of the power supply being severely heated. The same applies if the mains voltage supply is interrupted with the power supply being connected. In that case there is still a slowly decreasing supply voltage present across a capacitor that is used to smooth the mains voltage, as a result of which the effects mentioned, poor regulating behavior and imminent heating, are likewise brought about.
In order to measure the supply voltage, known power supplies are provided with a voltage divider, which divides the supply voltage down to a few volts, and a comparator is provided for comparing the divided-down voltage with a reference value. For reasons of fire protection, the voltage divider must be constructed from three resistors, two of which must be resistant to high voltages. The result of the comparison has to be fed to a control circuit of the switched-mode power supply, the control circuit usually being accommodated as an integrated circuit in an IC housing, for which one connection pin of the IC housing is xe2x80x9cused upxe2x80x9d. Furthermore, the external components of the voltage divider, that is to say the components situated outside the control circuit, constitute a cost factor. Moreover, the voltage divider consumes a not inconsiderable power serving only for measurement purposes.
It is accordingly an object of the invention to provide a switched-mode power supply and a method for determining the supply voltage in a switched-mode power supply that overcome the above-mentioned disadvantages of the prior art devices and methods of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a switched-mode power supply, including:
a transformer having a primary coil;
a switch for providing a pulsed application of a variable supply voltage to the primary coil of the transformer;
a current measuring configuration providing a current measurement signal dependent on a current through the primary coil; and
an evaluation circuit connected to the current measuring configuration, the evaluation circuit evaluating a time characteristic of at least one of the current through the primary coil and the current measurement signal and the evaluation circuit determining a value of the variable supply voltage.
According to the invention, the evaluation circuit, which is connected to the current measuring configuration, evaluates the time characteristic of the current through the primary coil and/or of the current measurement signal. The evaluation circuit is provided for the purpose of determining the supply voltage and has an output terminal at which a signal dependent on the supply voltage can be tapped off. In this case, the invention makes use of the fact that after the switch has closed, the characteristic, in particular the rise with respect to time, of the current through the primary coil is dependent on the supply voltage. Shortly after the switch has closed, the current rises approximately linearly in a manner dependent on the supply voltage and the inductance of the primary coil. The evaluation circuit assesses this rise and generates an output signal which is dependent on the gradient of the current signal, and hence on the supply voltage. The evaluating circuit can be completely integrated in a control circuit without external components. The current measurement signal fed to the evaluation circuit for the purpose of determining the current rise is usually fed to the control circuit of a switched-mode power supply in any case for the purpose of generating a drive signal for the semiconductor switch. Therefore an additional connection pin is not necessary either.
One embodiment of the invention provides for the evaluation circuit to have a switching device, for providing a signal dependent on a gradient of the current measurement signal. In addition to the constant conductance of the primary coil and, if appropriate, resistors connected into the primary current circuit, this signal is dependent on the supply voltage and thus represents a measurement signal for the supply voltage.
In accordance with a further embodiment, provision is additionally made of the switching device to compare the signal dependent on the gradient of the current measurement signal with a reference signal. In this case, a two-value signal is preferably available as the output signal, said two-value signal assuming one of the two values depending on whether the supply voltage exceeds or falls below a predetermined value. The evaluation circuit thus serves for identifying an excessively low supply voltage.
One embodiment of the invention provides for the evaluation circuit to have a series circuit formed by a further current measuring configuration and a capacitance element. The series circuit being connected to the current measuring configuration in such a way that a voltage representing the current measurement signal is present across the series circuit. For generating the current measurement signal, the first current measuring configuration preferably has a resistor connected in series with the primary coil. If the current through the primary coil rises after the switch has closed, the voltage across the resistor and across the series circuit formed by the second current measuring configuration and the capacitance element rises proportionally thereto. The rise in this voltage causes a current to flow to the capacitance element, the current flowing to the capacitance element being proportional to the derivative of the voltage characteristic with respect to time. In the case of a linearly rising voltage, the current is constant and, in addition to the constant values of the inductance of the primary coil and of the resistor of the first current measuring configuration, is dependent on the supply voltage. The current is larger, the larger the supply voltage is, that is to say the more steeply the primary current rises.
A further embodiment of the invention provides for a current source to be connected to a node which is common to the capacitance element and the further current measuring configuration, and to evaluate the direction of the current flow through the further current measuring configuration for the purpose of assessing the supply voltage. In this case, the current direction is dependent on the gradient of the current through the primary coil and/or of the current measurement signal. If the current supplied by the current source is smaller than a current which flows to the capacitance element on account of the rise in voltage across the series circuit, another current flows from the current measuring configuration via the further current measuring configuration to the capacitance element. If the current supplied by the current source is larger than a current which flows to the capacitance element on account of the rise in voltage across the series circuit, a current flows in the opposite direction from the current source via the further current measuring configuration to the current measuring configuration.
The further current measuring configuration preferably has a resistor, with terminals to which a comparator is connected for the purpose of determining the current direction. One of two different signals is present at an output of the comparator, depending on the direction in which current flows through the resistor. A flip-flop is preferably connected to an output terminal of the comparator, for storing a signal present at an output at the comparator according to drive pulses. The drive pulses are preferably provided by a pulse width modulator for the driving of the switch, and ensure that the signal at the output of the comparator is stored each time the switch is opened.
A switch is preferably connected in parallel with the capacitance element, which switch is driven by drive pulses in order to discharge the capacitance element before the closing or after the opening of the switch.
The invention furthermore relates to a method for determining the supply voltage in a switched-mode power supply, the method provides for the evaluation of the time characteristic, in particular the rise, of the current through the primary coil after the semiconductor switch has closed. In accordance with one embodiment, the evaluation is effected by the determination of a gradient of the current measurement signal and by the comparison of the gradient with a reference value.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a switched-mode power supply and a method for determining the supply voltage in a switched-mode power supply, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.